1. Field of the Invention
The present invention relates to a system for detecting an oxygen concentration in exhaust gas of an automotive engine, and more particularly to an oxygen concentration sensing device for use in an air fuel-ratio feedback control system for an automotive internal combustion engine.
2. Description of Background Information
Air-fuel ratio feedback control systems for an internal combustion engine are generally constructed such that the oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor and the air-fuel ratio of the mixture to be supplied to the engine is feedback controlled in response to a result of the detection of the oxygen concentration so as to purify the exhaust gas and improve the fuel economy.
As an example of an oxygen concentration sensing device for use in an air-fuel ratio control system of this type, an oxygen concentration sensing device having an output signal whose level is proportional to the oxygen concentration in test gas (whose oxygen concentration is to be measured) is described in Japanese patent application laid open No. 58-153155. This oxygen concentration sensing device has a sensor element whose general construction includes a pair of flat solid electrolyte members having oxygen ion permeability. These solid electrolyte members are placed in the atmosphere of the test gas. Further, two electrodes are provided on the front and back surfaces of both of the solid electrolyte members. These two solid electrolyte members each having a pair of electrodes are arranged in face to face relation with each other to form a gap portion between them.
With this arrangment, one of the solid electrolyte members serves as an oxygen pump element and the other one of the solid electrolyte members serves as a cell element for sensing an oxygen concentration ratio. A drive current is supplied accross the electrodes of the oxygen pump element in such a way that the electrode facing the gap portion is supplied with the negative current in the atmosphere of the test gas. By the supply of this current, the oxygen gas component of the gas within the gap portion is ionized on the surface of the negative electrode of the solid electrolyte member serving as the oxygen pump element. The oxygen ions migrate through the inside of the oxygen pump element to the positive electrode, where they are released from the surface of the positive electrode in the form of the oxygen gas.
While this movement of oxygen ions occurs, an electric potential is generated across the electrodes of the solid electrolyte member operating as the cell element because the oxygen concentration is different for the gas in the gap portion and the gas outside the electrodes of the cell element. This difference of the oxygen concentration is caused by a reduction of the oxygen gas component within the gap portion. Then, if the magnitude of the electric current supplied to the cell element is varied so as to maintain the potential across the cell element, the magnitude of the electric current varies substantially linearly in proportion to the oxygen concentration of the test gas at room temperature.
FIG. 1 shows an example of a conventional oxygen concentration sensing device of the oxygen concentration proportional type. In this system, an oxygen concentration sensor which is generally designated at 1' includes a cell element 2' which is provided with a pair of planar electrodes 2a' and 2b'. An electric potential developed across the electrodes 2a' and 2b' is supplied to a differential amplifier 4 to which a predetermined reference potential V.sub.r (40 mV, for example) is also supplied. The differential amplifer 4 produces an output signal whose level varies with a difference between the electric potential of the cell element 2' and the reference potential. The output signal of the difference amplifier 4 is in turn supplied to a V/I converter 5 whose input terminal is connected to an output terminal of the differential amplifier 4. The V/I converter 5 supplies an electric current whose magnitude varies with the output signal level of the differential amplifier 4, across the electrodes 3a' and 3b' of the oxygen pump element 3'. The magnitude of the current supplied to the oxygen pump element 3' is detected by a current detection circuit 6, and the detected magnitude of current is then utilized as an oxygen concentration detection output signal.
The operation of this conventional device is such that an electric current I.sub.P is supplied from the V/I converter 5 to the oxygen pump element 3' so that the electric potential developed at the cell element 2' becomes equal to the reference potential. Therefore, an output signal whose level is proportional to the oxygen concentration of the test gas which is typically shown in FIG. 2 of the accompanying drawings can be obtained by detecting the magnitude of the current from the V/I converter 5.
However, in this type of conventional oxygen concentration detection device, a problem was that the structure of the system tends to be complicated, because in addition to the oxygen concentration sensor, various components such as the differential amplifier, the V/I converter, and the current detection circuit are required in order to produce an output signal whose level is substantially proportional to the oxygen concentration. Further, for use in an air/fuel ratio control system, the device requires comparing means for comparing an output signal level of the current detection circuit with a level corresponding to a target air/fuel ratio.